In modern electrical engineering, a wide variety of devices are used, some of which are designed to automate technological operations. Such is the stepper motor. The principle of operation and the device of this device are described in the article.
What it is?
This is the name of an electromechanical device that serves to transmit a control signal to the
mechanical movement of the rotor. Each movement ends with a fixation in a strictly specified position. The device can be angular or linear. It is worth remembering that a stepper
motor, the principle of which will be described below, is a synchronous device.
Open-loop control systems (open loop)
Most often, this equipment is controlled by a special electronic circuit. It is powered only by an AC source. Such engines are often used in circuits where speed control is required. This avoids the need to use an expensive and complex feedback loop, and motor protection becomes simpler (you only need to provide quick blackout).
This principle of operation is used in open-loop circuits. It should be remembered that this scheme (without a feedback loop) is beneficial from an economic point of view, but it has a number of significant limitations.
So, the rotation of the rotor is quite unstable, oscillatory, which is why the speed and other characteristics of the movement in no case can be as accurate as they are in DC motors with feedback loop. To expand the scope of the stepper motor, it is required to find ways to reduce vibration.
system configuration
To better understand the device of the stepper motor and the principle of its operation, we can consider the operation of the device under its control, which was used 20 years ago for the manufacture of punch cards. For this purpose, three- and four-phase SD were universally used. Now we will consider the scheme of the first.
We have already mentioned that the rotor of the engine rotates a certain distance in response to each control pulse. The value of this rotation is expressed in degrees and is called a step. The logic circuit is switched on when the signal is received, after which it immediately determines the phase necessary for activation. After that, it sends its signal to the inverter, which is responsible for the current value that the stepper motors use. The characteristics of this equipment involve the use of various types of control circuits. As a rule, the latter are mounted from widespread transistors, although relatively recently used
integrated circuits for this purpose
. With its high output potential, the necessary phase of the winding (first, for example) is automatically excited. If the potential decreases, this phase is automatically switched off. This is how motor protection is implemented.
Phases are designated by serial numbers 1, 2, 3, etc. either the letters A, B, C, etc. The latter option is used only in the case of some two-phase motors. Thus, at each particular moment in time, only one phase of two, three or four available (depending on the type of engine) is excited. When explaining the principles of operation of such a device, this circumstance is constantly mentioned, but it must be understood that this scheme is not an ideal control method at all.
Step and increment
The simplest option is to supply single pulses from the control circuit. In this case, for example, the engine at one time rotates the drive sprocket of the conveyor a certain distance forward. It should be noted that when the massive mechanism is fed forward only one step, the problem of vibration is further aggravated, and significant inertia makes itself felt.
In such cases, it is much more justified to use a stepper motor, which can make several movements in one control impulse. It also does not hurt to use an asterisk with smaller teeth. By the way, every such movement is called an increment.
In the cases described by us, the increment is equal to one or several steps, respectively. After each cycle, the engine stops for a while, after which everything is repeated again. This is called incremental motion and incremental control, respectively.
If one movement is performed in a few steps (as we discussed above), and there may be no oscillation of the rotor. When the movement is one-step, the vibrations have to be suppressed using a special electronic device. In general, stepper motors (the characteristics of which we are considering) relate to high-tech devices; for their work, a lot of complex electronic “stuffing” is required.
General management principle
For one increment, the number of steps over four falls in some production lines, conveyors. When data from a storage device (internal flash memory, computer hard drive) is sent to the controller, they are executed block by block. Each of them contains a strictly defined number of characters (32, 48 or 64), and in different systems and for different purposes of the device, this figure can seriously vary.
It is not surprising that in recent years, homemade products based on the Arduino microcomputer have become common. A stepper motor in this design is ideal, since in such a bundle it can be adapted both as a power unit for a toy, and for rather complex industrial equipment.
Before using it, the data block is transferred to the semiconductor memory on the controller, after which the movement will begin in accordance with the instructions that were recorded in the first information block (before connecting the electric motor, it is necessary to find out these characteristics).
After following the instructions, the system begins to read a second array of information. If each movement consists of many small steps, then an additional cascade must be mounted in front of the main controller. Most often, its functions are performed by the input controller. It sends data to the second control circuit at some interval specified by the system (Arduino). The stepper motor in this case is protected from overload by requests.
Some specifics of using SD
We will tell you about some of the nuances of using stepper motors, as well as define the terms that are often used in this area:
- A small step angle. As you already know, after each control pulse, the motor rotor rotates by a certain degree. The smaller the pitch, the higher the direct speed can be. It is important to know that stepper motors may very well provide a very small step. The step number in this case is the number of revolutions in one step, and this value is very important for engineers. It is calculated by the following formula:
S = 360 / θS, where S is the step number, θ is the step angle (rotation angle).
In most cases, the stepper motor drive can perform 96, 128 or 132 steps per revolution. Four-phase models sometimes have a value of 200. Rare types of precision motors can make 500 or 1000 steps at once. However, for simple varieties this is unattainable, since they have a rotation angle of 90, 45 or 15 °.
- High speed accuracy. It is this parameter that determines the overall quality of the device. You already know that the operation of a stepper motor involves stopping and fixing it in a certain position after the data block is executed. Of course, ordinary mechanics unambiguously tells us that due to inertia, friction force and other factors, all sorts of deviations from the given parameters are possible.
Adverse Event Management
The gap between the rotor and stator teeth is always minimized to increase the rigidity of fixation. The accuracy of positioning itself depends on the characteristics of the inverter only, since other factors affect it to a much lesser extent.
And now it is necessary to consider a number of important characteristics and concepts, such as the maximum static moment, the position of the "dead" rotor, as well as the accuracy of positioning of all these positions. To define the above terms, there are two generally accepted common concepts.
Maximum static effect
As we already said, it has two positions at once:
- Restraining. This is the maximum allowable effect, which theoretically can be applied to the shaft of an already excited stepper motor without movement.
- Latching. Accordingly, this is also the maximum static effect, which theoretically can be applied to the shaft of an unexcited engine without subsequent rotation.
The higher the holding moment, the lower the likelihood of positioning errors caused by unpredictable loads (capacitors for electric motors, for example, failed). Full locking torque is only possible on engine models that use permanent magnets.
Dead rotor positions
There are just three positions in which the rotor stops completely:
- Equilibrium position. It is a complete stop of the excited stepper motor.
- Fixation. Also the state in which the rotor stops. But this concept is used only in relation to those engines that have a permanent magnet in the design.
- In modern models of stepper motors that comply with all environmental and energy safety standards, the winding is completely de-energized when the rotor stops.
About positioning accuracy
Finally, let's talk about the most important concept. It's about positioning accuracy. You can guess how important it is in the operation of complex industrial equipment. There are two important terms:
- Angular position error. It is defined as a positive or negative departure from the normative angular state, which is very often observed in cases where the rotor moves from one position to another. As a rule, inertia is to blame, as well as poor fitting of parts.
- Positioning accuracy. This is the maximum value of the errors of the angular position of the rotor that arise over the entire period of the step motion.
Important! You can find the regulatory information for each category of stepper motors both on the official page of their manufacturers, and from the reference documentation that is attached to such products. As a rule, the error value ranges from +0.08 to -0.03 °. Simply put, positioning accuracy is calculated as the sum of these two indicators: 0.08 ° + 0.03 ° = 0.11 °.
Thus, the stepper motor, the principle of which we describe, refers to high-precision equipment.
High ratio of electromagnetic moment to moment of inertia
As you already imagine, a stepper motor is required to start moving as quickly as possible immediately after a control pulse arrives at the controller. It should stop just as quickly, with high positioning accuracy. If during the movement the sequence of control pulses is interrupted, the engine will stop working in the position determined by the last pulse.
It should also be borne in mind that the ratio of the electromagnetic moment to the moment of inertia of the rotor in the SD should be much higher than that for conventional electric motors.
Step speed and pulse frequency
Since the rotational speed of a speed wheel actually represents the number of steps per unit time, instead of the term “rotational speed”, in the specialized literature one can often find the definition “stepping rotational speed”. Before you connect the electric motor, you should definitely read about these nuances.
Since for most stepper motors this frequency is equal to the number of control pulses, one should not be surprised at its unusual designation in technical manuals. More precisely, for such motors, the unit of measurement is often hertz (Hz).
It is important to understand that the step frequency of rotation of the actual number of revolutions of the rotor of the engine in no way reflects. Experts believe that there is no reason not to use the same number of revolutions per minute in the description of stepper motors, which is used to describe the technical characteristics of conventional electric motors. The ratio between the real speed of rotation and its step counterpart is calculated by the following formula:
n = 60f / S, where n is the rotational speed, expressed in revolutions per minute; f is the step frequency of rotation; S is the number of steps.
By the way, how to determine the required capacitors for electric motors? Very simple! Just use this formula:
C = 66
It is easy to guess that Rn is the rated power of the electric motor in kW.
The simplest motor connection diagram EM-178
And now we will consider the simplest connection of a stepper motor using the example of the EM-178 model, which is widely used in industrial printers.
Phase 0 | White controller |
Phase 1 | Orange |
Phase 2 | Connecting the stepper motor to the red controller |
Phase 3 | Connects to a blue connector |
General "+" power | Brown controller |
It will simply not be possible to paint the work on a larger scale, since there are millions of the most diverse models, the characteristics of which have significant differences.
Currently, various types of electric motors of this design are used. In the article we will discuss the most common.
Jet engines
It is this variety of devices that is widely used to this day. In fact, this is an almost standard three-phase motor, on the stator of which there are six teeth. Simply put, every two teeth opposing each other belong to the same phase. Serial or parallel connection of their coils is used.
As for the rotor, only four teeth are located on it. Most often, manufacturers make the stator and rotor from soft magnetic material, but often you can just see massive rotors from ordinary metals. The thing is that there is only one important requirement for the substances that go into their production: they must provide the best conductivity of the magnetic field. This is extremely important when discussing a stepper motor: the principle of operation is directly related to the magnetic field strength.
Permanent Magnet Devices
A cylindrical magnet is used as a rotor, while the stator has four teeth with an individual winding. In order to reduce the pitch angle more strongly, in these models of stepper motors it is necessary to increase both the number of rotor poles and the number of teeth on the stator. However, it should be remembered that both of these parameters have rather strict physical limitations. In the last paragraph of our article there is information about their alternative design (bipolar stepper motor), but such models can be found not so often.
As we have already said, stepper devices with permanent magnets stop in a strictly fixed position even in cases when the voltage is removed from the windings. In this case, the fixation mechanism that we discussed above is triggered - the fixation position.
The use of permanent magnets is justified from many points of view, but at the same time, their use can lead to several problems at once. Firstly, their price is far from affordable. By the way, how much does such a stepper motor cost? The price of models with permanent magnets exceeds 100 thousand rubles.
Secondly, the maximum magnetic field density may not be too high, since this value is limited by the magnetization of the carrier itself. So, relatively cheap permanent ferrite magnets do not allow to obtain a more or less sufficient field strength. And what other types of electric motors are working on this principle?
Hybrid Installations
There is another type of stepper motor, partially using the same principle. Hybrid models operate using both jet and
magnetic motors.The rotor has almost the same design as a jet motor, but the windings are made according to a slightly different scheme. The fact is that at each pole only one coil has a winding (three-phase ).It is easy to guess that two coils are already wound in four-phase models. Winding is carried out according to a bifilar scheme. The peculiarity is that upon excitation on the coils a magnetic field of different polarities is created (bipolar stepper motor).